The control apparatus according to the present invention finds general utility where one, or a plurality of direct current electric motors, must have a predetermined drive performance under all operating conditions, in particular irrespective of load, or speed changes.
The factors affecting the speed-load characteristic of a direct current motor are as follows:
1. IR drop in the armature circuit. PA1 2. Series-field effect. PA1 3. Current flowing in short circuited armature coils caused by interpole over compensation or brush shifting. PA1 4. Armature distortion. PA1 5. Interpole flux leakage. PA1 6. Interpole saturation. PA1 To = mechanical time constant PA1 J = (WK.sup.2)/g PA1 a = volts per radian = V/.omega. PA1 b = torque per ampere = T/I
All of these factors are non-linear except the IR drop. It is desirable, though, that the speed-load characteristic be linear, for at least two reasons. (1), First, when regulating the speed of a direct-current motor, since the armature current is a function of the load, control of the speed and function of the armature current should be free from the non-linearities introduced by the above-mentioned factors, other than the IR drop. (2) The second reason for seeking linearity in the speed-load characteristic is that the mechanical time constant of the machine is a function of the torque, e.g. in close approximation of the load. If non-linearities are introduced, the mechanical time constant will vary and regulation will no longer be proportional to the flux reference.
These two points will be examined hereinafter, successively.
1. In order to improve, or define, the speed-load characteristic, a series-field is often used. The cumulative series-field provides a stabilizing effect by creating a drooping speed-load curve. Sometimes much series-field is required in order to get a drooping curve extending all over the intended range of regulation. Therefore at low speed (full field and reduced voltage) the series-field has very little effect. The droop and stall point, at low speed, are determined almost entirely by the IR drop of the motor. Also the calculated speed-load curves are usually more nearly linear than the actual test curves. In fact, the rapid droop in speed occurring for the first 25% of the load cannot be calculated. This rapid droop is thought to be due to the inability of the brushes to get a stable or ultimate contact drop established. Inconsistent circulating currents built-up among the armature coils being commutated at light loads are also accountable for this difficulty in determining the machine characteristic.
In addition, as a result of the lack of linearity, unusual speed rises occur in the speed-load characteristic curve from approximately 1/4 load to twice load. These are also difficult to calculate. The factors affecting such non-linearities are unrelated and cannot be foreseen from one machine to another. They are due to such effects as: 1. Interpole saturation; 2. Interpole flux leakage; 3. Incorrectly located brushes-spacing and neutral position; and 4. Circulating current under the brushes.
It is desirable always to have motor speed-load characteristics which are drooping. While a series-field can be used to insure droop when rotation is unidirectional, in reversing drives the series-field must be reversed when the load is reversed. This makes the use of a series-field unpractical on reversing drives.
From the preceding it appears that while a determination of the speed-load characteristic by calculation of the desired droop, and all the more so its linearity, is not easy, the use of a series-field winding in generating such compound effect is not practical, nor desirable.
2. Another source of difficulty in regulating a DC motor drive stems from the mechanical time constant of the machine which is as follows: EQU To = JR/ab
where:
The mechanical time constant can also be expressed as follows: ##EQU1##
In this form it appears that the mechanical time constant is a function of the slope of the speed-torque curve. This function is not quite as shown by equation (1) since the flux changes in the motor do not react instantaneously. However, the fact that the mechanical time constant varies woth torque cannot be ignored, and on the basis of equation (1) it may be anticipated that the flux changes occurring in the motor which as seen earlier disturb the regulation will also have a direct effect on the mechanical time constant. If a motor has a characteristic which is flat, the mechanical time constant will drop to zero. If the speed-torque curve is non-linear, the mechanical time constant will vary as the slope of the tangent to the curve at the operating point. For example, assuming a flat, or somewhat rising characteristic, as load increases demagnetizing effects decrease the flux. If the flux changes takes place instantaneously, or have zero time constant, then, the mechanical time constant is zero, or negative. If the voltage is increased on the motor armature the motor will try to accelerate instantaneously. As current flows, the flux will be progressively weakened and the ability of the motor to accelerate progressively impaired. The current will rise until the overload relay trips or the motor flashes over.
Another practical example of the same type of difficulty, is where the load connected to the motor has a very high inertia, e.g. introduces a very long mechanical time constant. If, at the same time, the motor field time constant which delays the demagnetizing effects is very short, we would have essentially the same adverse results. If acceleration is called for, the motor current will rise dangerously.
A contrasting example can be found with a motor which is stable so that an increase in load will result in either no flux change, or in an increase in flux. In such case, the accelerating current for a slight voltage change will depend on the increase in voltage and the regulation of the motor. No excessive armature currents will occur.
While the preceding examples relate to drive systems without consideration of regulatory loops, it should be appreciated that when regulators are used the mechanical time constant is directly in the regulating loop of any speed or position regulating system. If the motor is unstable, or if the regulation curve is not linear, the mechanical time constant will change and control of the operation will be affected.
The situation encountered in practice is complex because the IR drop acts instantaneously, while the flux changes are delayed by the main field time constant. In fact, the effect of flux changes due to load will vary, depending on the system used and the relative values of the time constants stants involved.
Accordingly, the present invention provides control apparatus for direct current motor drive which overcomes these shortcomings. According to the present invention, there is provided a control apparatus causing the regulation of a direct current motor in relation to the load to be substantially linear, whereby flux changes due to load changes are minimized and in which the mechanical time constant is the same under all operative conditions. As a result changes in load, or speed, will not upset the system constants or system relations, and the motor drive will perform consistently as set. This is highly desirable.
The object of the present invention is to regulate the flux of a direct current motor with a linear speed-load characteristic.
Another object of the present invention is to eliminate non-linear effects of load changes in the regulatory loop of a field-controlled direct current motor.
A further object of the present invention is to regulate the speed of a field-controlled direct current motor under constant flux for a predetermined speed level of control.